1. Field of the Invention
This invention relates to a light emitting device.
2. Background Art
Demand for light emitting devices in the visible wavelength range having high brightness and high emission efficiency is growing in illumination apparatuses, display apparatuses, traffic signals and the like.
A light emitting layer made of InGaAlP-based semiconductor can emit visible light in the green to red wavelength range. A lattice constant of InGaAlP-based semiconductor has a small mismatch with a lattice constant of GaAs. Hence, a GaAs substrate is often used as a substrate for crystal growth of a multilayer body including a light emitting layer. This serves to obtain a highly reliable light emitting device with reduced crystal defects.
On the other hand, GaAs absorbs visible light, and hence tends to decrease the brightness. The multilayer body can be transferred onto a translucent substrate, or bonded to another substrate via a reflective metal layer, to reduce optical absorption in the substrate and facilitate increasing the emission efficiency. However, with the increase in the planar size of the light emitting layer, the emission efficiency decreases due to absorption of light passing through the light emitting layer.
JP-A-2004-247635 (Kokai) discloses an example of a light emitting device in which emitted light can be extracted outside with high efficiency. This example includes a selectively provided light emitting layer and a high-resistance current block layer provided around this light emitting layer. The high-resistance layer concentrates the current on the light emitting layer and increases the emission efficiency. Furthermore, the light extraction efficiency can be improved by placing the light emitting layer near the center of the light emitting device.
However, in this example, the light emitting layer having high current density is located below a pad electrode. Hence, the pad electrode blocks the emitted light, causing the problem of decreased brightness.